1. Field of the Invention
The present invention relates to a method and apparatus for data transmission in RFID and remote sensor systems including at least one reader and one or more transponders or remote sensors that are located in an electromagnetic field of the reader, in which a multipart return link header containing transmission parameters for the return link, such as modulation references, is transmitted at the beginning of a return link transmission of useful data from a transponder or sensor to a reader.
2. Description of the Background Art
Automatic identification methods, also called auto-ID, have been widely used in recent years in many service sectors, in acquisition and distribution logistics, in commerce, in production, and material flow systems. A goal of auto-ID is, for example, the thorough provision of information on persons, animals, objects, and products.
An example of such auto-ID systems are chip cards, which are widely used today, and in which a silicon memory chip by mechanical-galvanic contacting using a reader is provided with power, read out, and optionally also is reprogrammed. In this case, the acquisition device is routinely called a reader, regardless of whether data can only be read thereby or also rewritten.
In RFID systems, the data carrier, e.g., the transponder, can be supplied with power not only through galvanic contact but also contactless with the use of electromagnetic fields within the radio range (radio frequency: RF).
RFID systems typically have two basic components, namely, the transponder or sensor in the case of a remote sensor system, i.e., an application-specific integrated circuit (IC) with a coupling element, such as a dipole antenna for transmitting and receiving, and of the reader (also: base station), which typically has a high-frequency module (transmitter-receiver) and also a coupling element. The reader provides the transponder or sensor, which usually does not have its own power supply, with power and a clock signal. Data are transmitted both from the reader to the transponder (forward link) and also in the opposite direction (return link).
Such RFID systems, whose range is considerably greater than 1 m, work with electromagnetic waves in the UHF and microwave range. In this case, a backscattering method, typically called the backscatter principle because of its physical operating mode, is used predominantly, during the course of which a portion of the energy arriving at the transponder from the reader is reflected (backscattered) and in so doing can be modulated for data transmission. The IC receives via the coupling element a high frequency carrier, which it transmits by means of suitable modulation and backscattering devices partially back to the reader.
The RFID and remote sensor systems, outlined above and based on backscattering, generally have the disadvantage that the return link is very weak with respect to the power balance, primarily because of the free space attenuation both in the forward and return link. For this reason, attention must be focused especially in the design of such systems that a high signal-to-noise ratio (SNR) and thus a low bit error rate can be achieved.
A possible approach is the use of so-called “synchronous return links,” in which the reader at certain time intervals sends synchronization tags (notch signals), which define a bit length in the return link. Thereby, additional expenditure for circuitry is usually necessary, which has an unfavorable effect on the price of such systems, for example, due to the use of special processors, such as digital signal processors (DSP). Several prior-art systems, such as the system developed within the scope of the Palomar project (Friedrich, U., Annala, A.: Palomar—a European answer for passive UHF RFID-applications. RFID Innovations 2001 Conference, London), are based on a synchronous return link.
Moreover, during use of synchronous return links an interfering effect on other readers in the vicinity due to the unfavorable power balance is disadvantageous. In this case, the difference between the sender and receiver can easily constitute 100 dB; i.e., only a level of about −70 dBm still occurs at the receiver. If in addition other sending or even modulating readers (production of notch signals) are in the vicinity, this circumstance has an especially interfering effect.
ISO standard 18000-6 FDIS, furthermore, describes systems with an asynchronous return link, in which a transponder or sensor transmits a “free” data stream without being affected by synchronization tags sent by the reader. Such asynchronous link mechanisms can be realized in UHF RFID systems more economically than the named synchronous link mechanisms, because, for example, normal processors can be used instead of DSPs. Nevertheless, in this connection, the SNR values as well are poorer in comparison with the synchronous solution, which can be acceptable, however, if the effect of reflections primarily in the near range, e.g., multipath propagation effects, is low.
Asynchronous methods possess advantages during use in RFID or remote sensor systems, which comprise a plurality of readers within a common range, because the noise contribution can be reduced by asynchronous operation. A synchronous return link is to be preferred, however, in cases of only a small number of readers or merely an insignificant effect for some other reasons.
Another inherent disadvantage of synchronous links is that suitable time bases on the transponder chip must be very accurate or suitably long synchronization character strings are required.
In the prior-art methods, it is regarded as particularly disadvantageous that these are defined in each case for a single link mechanism (cf. ISO 18000-6; Palomar). A possible workaround could be an appropriate expansion of the instruction set of known solutions related to the critical command and parameter fields. Nevertheless, the result of such an approach would greatly increase the amount decoding, which would have a negative effect on system efficiency.